Hard water can cause stains on articles as a result of a visible film depositing onto the surface of the articles. The film may be caused by calcium present in hard water precipitating and depositing onto the surface. To prevent such precipitation, cleaning compositions can include a chelating agent.
In some circumstances, precipitation of calcium salts can be beneficial. Etching or corrosion of glass or aluminum is a common problem in warewashing and surface cleaning. Glassware that is repetitively washed in automatic dishwashing machines has a tendency to develop an etching problem as evidenced by a surface cloudiness that is irreversible. The cloudiness can manifest itself as an iridescent film that displays rainbow hues in light reflected from the glass surface. The glass becomes progressively more opaque with repeated washings. It is believed that the glassware corrosion problem relates to two separate phenomena; the first is corrosion or etching due to the leaching out of minerals from the glass composition itself together with hydrolysis of the silicate network, and the second is chelation of ions contained in the glass by the detergent's builder.
Common corrosion inhibitors work by causing controlled precipitation of calcium salts, which can reduce such etching or corrosion. It is counterintuitive to include a second hardness ion (e.g., magnesium ion) with silicate or carbonate to achieve a corrosion inhibitor which does not cause scaling as an undesirable side-effect.